1. Field of the Invention
The present invention relates to a thermal recorder, and more particularly, to an improvement of a thermal head bar support construction of the thermal recorder in view of the positioning of a thermal head bar relative to a platen roller.
2. Description of the Prior Art
In a thermal recorder having a thermal head bar bearing a linear array of a number of heat generating elements and a platen roller between which a thermally recording sheet such as a thermally perforatable stencil or a thermally chromophoric sheet is clamped and transferred in a direction of traversing the array of heat generating elements according to a rotation of the platen roller such that an image is generated in the thermally recording sheet by a digital heat generating actuation of the respective heat generating elements according to an image signal, it is generally conventional that the platen roller is mounted to either of a housing of the recorder and a cover pivotably mounted to the housing to be pivotable between an open position and a closed position, and the thermal head bar is mounted to the other of the housing and the cover, wherein the platen roller and the thermal head bar are pressed to one another with the array of heat generating elements of the thermal head bar being aligned to a generatrix of a cylindrical surface of the platen roller when the cover is in the closed position, while the platen roller and the thermal head bar are detached from one another when the cover is in the open position.
In such a construction of the thermal recorder, the precision with regard to how the array of heat generating elements of the thermal head bar is correctly aligned with a generatrix of the cylindrical surface of the platen roller and the precision with regard to how the thermal head bar and the platen roller are longitudinally correctly positioned relative to one another are dependent upon the precision of the pivotal opening and closing movement of the cover relative to the housing. In such a thermal recorder, since the cover is opened for the purpose of charging a thermally recording sheet between the thermal head bar and the platen roller as well as cleaning and maintenance of the thermal head bar and the platen roller, in order to ensure the convenience of those operations, the radius of the pivotal movement of the thermal head bar or the platen roller in the opening and closing of the cover is generally designed to be at least 15 cm. In this case, if an allowance of errors in the manufacture of the covers and the assembling of the covers to the housings should be, for example, 0.2% of the radius of pivotal movement of the thermal head bar or the platen roller, when the pivotal radius is, for example, 15 cm, the absolute error in the radius of the pivotal movement would reach 0.3 mm. Such an error is very substantial, as it is approximately 10% for an image like characters of a height of a few mm.
For a high quality and uniform thermal image to be generated in a thermally recording sheet by clamping it between a thermal head bar and a platen roller and selectively activating each of the heat generating elements constructing the array thereof, it is very important that the array of heat generating elements is pressed against the cylindrical surface of the platen roller correctly along a generatrix thereof and there occurs no unstable shifting between the array of heat generating elements and the platen roller in the axial direction.